OSL `noise_turbulence` function
The reason that function does not come in the OSL Language Specification, it because 'noise_turbulence' is in fact a function done in OSL, and not hard coded into the OSL source.

It's located in the 'node_texture.h' (along with the voronoi function), and it can be found in the \blender\2.XX\scripts\addons\cycles\shader\ folder.